Start with the full capture.Select the bit you want to crop (in theory you can fade within, or outside, this selection - but with my settings at any rate, it always crops and fades outside the selection)

With Screenshot Captor, the dialog tomos showed is for splicing (with effects added).But it might be more straighforward to FIRST do a normal crop, and then from the SpecialFx2 menu, choose "Border" and then last submenu item is "border effect splice style (ragged effects)"which will give you this dialog:

Which is mostly the same but lets you choose what edges to apply the effect to.

ps.Fade out is ok, but i personally prefer the ragged edge effects when i want to indicate a crop:

With Screenshot Captor, the dialog tomos showed is for splicing (with effects added).But it might be more straighforward to FIRST do a normal crop, and then from the SpecialFx2 menu, choose "Border" and then last submenu item is "border effect splice style (ragged effects)"which will give you this dialog:

@tomos: Heh. Thanks. I hadn't realised the number had got up to 4,000. I hope the majority of them were helpful/useful/informative. Quite a lot of writing/typing there...I can only type like that because of Cu (Copper).